1. Field of the Invention
The present invention relates to a fixing device used for an electrophotographic image forming apparatus.
2. Description of the Related Art
Nowadays, fixing devices increasingly use a cylindrical film (also referred to as a belt) as a fixing rotating member. These fixing devices use a film so as to reduce heat capacity and power consumed by the fixing device. In some of the fixing devices using the film, a ceramic heater is in contact with an inner surface of the film or a halogen heater is used as a heat source.
Since the film has flexibility, in order to form a fixing nip portion in the fixing device using the film, a backup member is required. The backup member is in contact with the inner surface of the film and backs up the film from the inside of the film. Furthermore, in order to suppress bending of the backup member, which is caused by a load required to form the fixing nip portion and applied to the backup member, it is required that the backup member be reinforced by a metal reinforcing member (stay) serving as a beam. When the ceramic heater is used, the heater or a heater holder which is made of resin serves as the backup member. When the halogen heater is used, a molded component which is made of resin or a sheet-shaped backup member is provided between the reinforcing member and the film.
When continuously printing on small-sized recording media, as one of methods to suppress reduction in the number of sheets output per unit time, the width of the fixing nip portion in the recording medium conveying direction may be increased. When the nip width is increased, control target temperature (fixing temperature) can be correspondingly reduced during fixing of toner images. This can suppress an increase in temperature in a sheet non-passing portion of the fixing device where the recording media does not pass through. Since the increase in temperature in the sheet non-passing portion can be reduced, reduction in the number of sheets output per unit time can be suppressed. In order to increase the nip width of the fixing device using the film, it is required that the width of the backup member in the recording medium conveying direction be increased. When the width of the backup member is increased, it is required that a region of the backup member to be reinforced by the reinforcing member be increased.
In order to reduce the weight of the reinforcing member while obtaining the moment of inertia of area, a metal plate having been bent to have a U-shaped section is used in many fixing devices. When the ceramic heater is used in the fixing device, it is required that a sensor that monitors the temperature of the heater and a protection element (a temperature fuse or a thermo switch) that has a switching structure for cutting off power supply to the heater in an emergency be disposed on a rear surface of the heater. In order to arrange these elements on the rear surface of the heater, a through hole is provided in the heater holder. Furthermore, in order to route the wiring of these elements to the outside of the cylindrical film, leg portions of the U-shape of the reinforcing member are pressed against the heater holder so as to provide a space for routing the wiring. However, when the leg portions of the U-shape of the reinforcing member are in contact with the backup member for reinforcement, it is unlikely that a large region of the backup member is reinforced because of a contact region, where the reinforcing member and the backup member are in contact with each other, is small.
When a flat portion, which is a bottom portion of the U-shape of the reinforcing member, can be in contact with the backup member for reinforcement, a large region of the backup member can be reinforced. Thus, such a structure is suitable for increasing the nip width. However, as described above, when the ceramic heater is used in the fixing device, the space is required for the elements disposed on the rear surface of the heater and for the wiring of the elements. Thus, it is unlikely to use a configuration in which the flat portion of the reinforcing member is pressed against the backup member.
Alternatively, a heat source such as a halogen heater may be used for radiating heat to the film. In this case, it is not required to provide the through hole for arranging the elements including the protection element in the backup member. Thus, the configuration in which the flat portion of the reinforcing member is pressed against the backup member can be adopted (Japanese Patent Laid-Open No. 2004-94146). With the method by which the film is heated by radiant light, a large region of the film in the circumferential direction can be heated. Thus, a time period taken to warm up the film to the temperature, at which fixing is possible, can be reduced.
However, the method by which the film is heated by the radiant light is used and the flat portion of the reinforcing member is pressed against the backup member as described above, the reinforcing member blocks the radiant light, thereby limiting a region of the film exposed to the radiant light. When the region of the film exposed to the radiant light is reduced, the temperature of the film required for fixing is unlikely to be obtained. Although the heating region can be increased by increasing the diameter of the film, this increases the heat capacity of the film.